The Applicant has developed a range of Memjet® inkjet printers as described in, for example, WO2011/143700, WO2011/143699 and WO2009/089567, the contents of which are herein incorporated by reference. Memjet® printers employ a stationary pagewidth printhead in combination with a feed mechanism which feeds print media past the printhead in a single pass. Memjet® printers therefore provide much higher printing speeds than conventional scanning inkjet printers.
In order to minimize the amount of silicon, and therefore the cost of pagewidth printheads, each Memjet® printhead IC is fabricated via an integrated CMOS/MEMS process to provide a high nozzle packing density. A typical Memjet® printhead IC contains 6,400 nozzle devices, which translates to 70,400 nozzle devices in an A4 printhead containing 11 Memjet® printhead ICs.
As described in U.S. Pat. No. 7,246,886, the contents of which are incorporated herein by reference, a typical printhead fabrication process for Memjet® printhead ICs requires etching of holes in a frontside of a CMOS wafer via DRIE (deep reactive ion etching), filling the holes with a sacrificial material (e.g. photoresist) to provide a planar frontside surface, and then subsequently building MEMS nozzle devices on the frontside of the wafer. After completion of the all frontside MEMS fabrication steps, the wafer is thinned from the backside and trenches are etched from the backside to meet with the filled frontside holes. Finally, all sacrificial material is removed from frontside holes and MEMS nozzle chambers by oxidative ashing. In the resulting printhead IC, the frontside holes define individual inlet channels for nozzle chambers.
A critical stage of fabrication is plugging the frontside holes with sacrificial material and planarizing the frontside surface of the wafer. If the frontside surface is not fully planar, then any lack of planarity is carried through subsequent MEMS fabrication steps and, ultimately, may lead to defective devices or weakened MEMS structures with shorter installed lifetimes.
One process for plugging holes formed by DRIE is described in U.S. Pat. No. 7,923,379. In this prior art process, a hole is filled in multiple stages by spinning on sequential layers of a photoresist. After each of these stages, the photoresist on the front surface of the wafer is selectively exposed and developed to leave only photoresist partially filling the hole. The remaining photoresist inside the hole is hardbaked and the process repeated until the hole is fully filled with photoresist. The aim is to provide a hole plugged with photoresist at the end of the process, whereby an upper surface of the photoresist plug is coplanar with a frontside surface of wafer. This is the ideal foundation for subsequent MEMS fabrication steps on the frontside surface of the wafer.
However, the process described in U.S. Pat. No. 7,923,379 has a number of drawbacks. Firstly, it is not possible to achieve true planarity at the end of the process, because the hole is usually slightly overfilled or underfilled after the final exposure and development steps. Secondly, photoresist is highly viscous, which inhibits the escape of solvent or air bubbles. Bubbles can escape from the relatively thin final layer of photoresist, but cannot readily escape from the layer(s) of photoresist at the bottom of the hole. During thermal curing, these trapped solvent bubbles may combine and expand to form relatively large voids, with consequent instability in the plug. Thirdly, photoresists typically contract during thermal curing (‘hardbaking’). Contraction of the photoresist during hardbaking also affects the stability of the plug. Thus, even if a planar upper surface can be achieved, the photoresist plug may be susceptible to ‘dishing’ during subsequent MEMS fabrications steps; and any lack of stability in the photoresist plug may lead to problems in subsequently constructed MEMS structures e.g. nozzle plate cracking.
Thermoplastic polymers, which typically have lower viscosities than most photoresists and can be reflowed when heated, offer a potential solution to at least some of the problems associated with trapped solvent bubbles and contraction of photoresist as described above. However, thermoplastic polymers are not usually photoimageable and require planarizing via a chemical-mechanical planarization (CMP) process. Although a CMP process is technically possible for thermoplastic polymers, it is not practically feasible for thick layers of polymer, which are required to fill relatively deep holes formed by DRIE. This is due to: (1) poor stopping selectivity on the frontside surface when planarizing thick layers of polymer; (2) the rate of CMP being unacceptably slow for large scale fabrication; (3) rapid ‘gumming’ of CMP polishing pads, which consequently require regular replacement.
It would be desirable to provide an alternative process for filling photoresist holes, which ameliorates at least some of the problems described above.